A telephone knows essentially two basic states, namely on-hook (the receiver is on the hook) and off-hook (the receiver has been taken off the hook). In the on-hook state, a constant voltage is output—there is no DC current flowing—by an SLIC (Subscriber Line Interface Circuit), and in the off-hook state a constant current is output. As a result, it is possible to detect the respective state of the telephone.
In the off-hook state, the interface (SLIC) behaves as a power source which drives a load which is composed essentially of the resistance of the transmission line and the impedance of the telephone. An equivalent circuit diagram of the interface, composed of a power source I0 with an output resistance RI, and a load resistance RL, illustrated in FIG. 1a. 
This equivalent circuit diagram applies to what is referred to as the constant current region A between 0V, and a break point voltage VK in which the current profile is relatively flat (cf. FIG. 1b).
In FIG. 1b, a DC characteristic of the current Ia,b which is supplied by the SLIC in the off-hook state is represented plotted against the voltage Va,b present at the overall load. The DC characteristic can be divided essentially into the constant current region A whose gradient is determined by the output resistance RI, and into a resistance region B whose gradient is determined by a resistance RR. The break point between the constant current region A and the resistance region B is determined by what is referred to as the break point current IK and the break point voltage VK. The maximum achievable voltage is designated by Vlim.
The DC characteristic is usually implemented as software.
In addition, a straight line b whose gradient corresponds to the load resistance RL is shown in FIG. 1b. The working point AP of the circuit occurs at the point of intersection of the section a and of the straight line b.
If a telephone set is closer to the switching office, the gradient of the straight line b becomes steeper (smaller line resistance) and the working point AP migrates in the direction of higher current values (arrow C). If, on the other hand, the line transmission between the office and telephone set is longer, the working point AP migrates in the direction opposed to that of arrow C.
As is apparent, the current flowing on the transmission line, and thus also the power loss, depends on the length of the transmission line, the power loss being greater with short lines than with long lines.
In order to reduce the power loss with short lines it would be theoretically conceivable to limit the current flowing on the transmission line by means of a higher output resistance RI. The relatively flat profile of the DC characteristic a could then be obtained in the constant current region A (arrow D). If there is an infinitely high output resistance RI, the current will be constant and equal to the break point current IK, in which case the line c shown by dotted lines will then be set. However, in practice it is not possible to execute the current source with an infinitely high output resistance.
To generate high output resistances it would also be conceivable to program the output resistance RI (for example in a range between 40 to 5 kOhm). High output resistances have the advantage that a DC regulator provided in a Codec (coder/decoder) for setting the working point AP settles more quickly. However, given extremely flat current profiles, this has a negative effect on the stability properties of the regulation. In addition, high output resistances RI influence the AC impedance of the circuit, and worsen in particular the reflections at the end of the transmission line and the frequency response.
If, on the other hand, the output resistance RI is set too small, the current continues increasing for short transmission lines, as a result of which the power loss continues increasing.